docs: add browser redirects configuration step and missing custom component instructions for Next.js quickstarts

[wassimoo]

Related Issue or Design Document

This PR introduces fixes and missing steps to the Next.js quickstart guide:

• Add browser redirect configuration verification step
• Add missing custom component code
• Add missing env variable required step

Checklist

• [x] I have read the contributing guidelines and signed the CLA.
• [x] I have referenced an issue containing the design document if my change introduces a new feature.
• [x] I have read the security policy.
• [x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got approval (please contact [email protected]) from the maintainers to push the changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added the necessary documentation within the code base (if appropriate).

Further comments